Alburger
process for fluorescence detection
of extremely thin tracer films



May 19, 1970 J. R. ALBURGER PROCESS FOR FLUORESCENCE DETECTION OFEXTREMELY THIN TRACER FILMS Original Filed OCT.. 4, 1965 nu 0 L.

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United States Patent O 26,888 PRCESS FOR FLUORESCENCE DETECTION FEXTREMELY THIN TRACER FILMS James R. Alburger, 5007 Hillard Ave., LaCanada, Calil". 91011 Original No. 3,386,920, dated June 4, 1968, Ser.No. 492,674, Oct. 4, 1965, which is a continuation-impart of applicationSer. No. 323,529, Nov. 13, 1963, which is a continuation-impart ofapplication Ser. No. 149,061, Oct. 31, 1961, which in turn is acontinuation-impart of application Ser. No. 82,374, Jan. 13, 1961.Application for reissue May 19, 1969, Ser. No. 830,903

Int. Cl. C091; 1/02; G0111 2.7/38 U.S. Cl. 252-3012 23 Claims Matterenclosed in heavy brackets appears in the original patent but forms nopart of this reissue specification; inatter printed in italics indicatesthe additions made by reissue.

ABSTRACT 0F THE DISCLOSURE A fluorescence inspection process for thedetection of extremely thin jlms of fluorescent tracers. Conventionalfluorescent tracer processes have utilized solutions of fluorescent dyeswhich exhibit dimensional thresholds of fluorescence response largerthan about 250 millimicrons. In newly dscoif'ered supersensitivefluorescent tracer processes, where it is desired to exhibit thepresence of thin layers of tracer material, smaller than about 250millimicrons, certain tracer compositions may be formulated using one ormore fluorescent sensitizer dyes, with or without color former dyes,dissolved in a carrier liquid at a concentration greater than about 15grams per pint, and the thus-formulated tracer compositions may beapplied to test surfaces for thin-film fluorescence detection.

RELA TED PA TENTS AND PA TENT APPLICATIONS U.S. Put. No. 3,107,298,issued Oct. l5, 1963, for Apparatus for the Measurement of FluorescentTracer Sensitii'ity.

U.S. Pat. No. 3,184,596, issued May 18, 1965, for Flaw Detection MethodUsing a Liquid Solvent Developer.

U.S. Pat. No. 3,311,479, issued Mar. 28,1967, for Penetront InspectionProcess and Compositions.

U.S. Pat. No. 3,320,417, issued May 16, 1967, for FluorescentImage-Forming Screen.

Application Ser. No. 728,458, filed May 13, 1968, for Method and Meansof Preventing fraud in Documents.

The present application is a continuation-in-part of my copendingapplication, Ser. No. 323,529, led Nov. 13, 1963, for FluorescentTraccrs, which latter application was a continuation-impart of myapplication, Ser. No. 149,061, filed Oct. 31, 1961, for FluorescentTracers, which last application was a continuation-inpart of myapplication, Ser. No. 82,374, filed Jan. 13, 1961, for FluorescentPenetrant Traccrs, all now abandoned.

The present invention relates to fluorescent tracers, and moreparticularly to fluorescent tracers having improved sensitivityperformance characteristics.

Fluorescent tracers are well known in the prior art, and have usuallybeen comprised of one or more lluorescent dyes suspended in a suitablecarrier material. These tracers have found advantageous usage inindustrial inspection processes. Thus, such tracers have been employedin the detection of surface aws in parts constructed of metal, ceramic,or other material. When used for this purpose, the fluorescent dye andcarrier is utilized in the form of a penetrant liquid which formsentrapments in the flaws and renders the latter more readily detectablethan might be the case with ordinary visible colored dyes.

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In the use of fluorescent penetrants for detection 0f extremely smallflaws, it has heretofore been thought that the ability of the tracer todetect the flaws is a function of its fluorescent brightness. Basically,fluorescent brightness, of course, depends upon the particularfluorescent dye or dyes used. However, such brightness can be enhancedby the `well known effect of cascading of uorescence. Whereas thebrightness effects produced by combinations of two or more fluorescentdyes are ordinarily approximately additive, cascading involves thetransfer of radiant energy from one dye component to another, with aconsequent increase in brightness which is more than merely additive.

Since, as has been pointed out, the Sensitivity of' a fluorescentsubstance as regards its ability to reveal its presence in microtraceshas been equated directly with lluorescent brightness, it has often beenthe practice in industrial inspection processes to attempt to maximizesuch sensitivity of the uoresccnt tracer used by increasing thetluorescent brightness thereof; e.g., through the use of theaforementioned effect of cascading of fluorescence. Thus, when a llawand its tracer entrapment are so small as to be virtually invisibleunder black light, it has usually been felt necessary to employ a tracerof increased iluorescent brightness so as to make thc flaw detectable.However, in spite of the attempts heretofore made to maximize thesensitivity of lluorescent penetrant materials by increasing thefluorescent brightness thereof, presently known fluorescent penetranttracers are unable to detect extremely small llaws which may result fromsuch effects as inter-crystalline corrosion or creep cracks, and whichmay have dimensional magnitudes on the order of 105 to 10-E centimeters.

The failure in the prior art to produce fluorescent tracers which areable to detect flaws of such small dimensional magnitude has resultedprimarily from the emphasis which has been laid on the fluorescentbrightness of the particular dyes employed, as discussed above. Thepossibility of improving fluorescent response through increase inconcentration of the dye used has not been given consideration. In fact,the prevailing belief is that any substantial increase in dyeconcentration is constraindiciated. Such belief is based on thegenerally accepted assumption that lluorescent brightness response tendsto be self-quenching when the fluorescent dye is used in a concentrationin solution above an approximate range of about 0.5% to 2% by weight.

In addition to the aforementioned shortcomings of known fluorescent dyeswhen used for certain applications in the penetrant inspection eld, suchdyes have also been unable to serve adequately for other detection ortesting purposes, or for marking, decoration, and other applications inwhich thin-film characteristics are desired.

The `principal object of the invention, therefore, is to provideimproved lluoresccnt tracer materials.

Another object of the present invention is to provide lluorescent tracermaterials having improved sensitivity performance characteristics.

Still another object of the present invention is to provide fluorescenttracers which exhibit sensitivity levels far greater than thatobtainable through enhancement of fluorescent brightness.

A further object of the present invention is to provide uorescenttracers having improved characteristics for testing, marking,decorative, and other purposes in which thin-film properties aredesirable.

A still further object of the invention is to provide fluorescenttracers for penetrant inspection purposes, capable of detecting flawshaving dimensional magnitudes within a range as low as 10-5 to 10M6 cm.or Smaller.

These and other objects of the invention will become more apparent fromthe following description thereof when read in conjunction with theaccompanying drawings, in which:

FIG. I is a chart employable in conjunction with the use of thefluorescent tracers of the invention, a reading of said chart for aparticular tracer being shown;

FIG. 2 is a diagrammatic representation of a surface flaw illustratingthe use of tracer materials of the invention;

FIG. 3 is a diagrammatic representation of a leakage flaw illustratingthe use of tracer materials of the invention; and

FIG. 4 is a diagrammatic representation of a thin film surface coatingillustrating the use of tracer materials of the invention.

The present invention provides fluorescent tracers, each of which isformulated essentially by the solution in a suitable solvent of afluorescent sensitizer selected from a group to be defined hereinbelow,and at least to a minimum level of concentration up to a maximum levelof concentration depending on the solubility limit of the sensitizer, aswill be described. In order to understand the reason for the particulargrouping of sensitizers made, and for the set minimum level ofconcentration, reference should first be made to the concept, which Ihave formulated, of thin-film fluorescence. In accordance with thisconcept, a fluorescent sensitizer, such as a fluorescent dye or otherfluorescent substance, when in solution exhibits the characteristic of athreshold of lm thickness below which fluorescence response ceases.Thus, for a given fluorescent dye dissolved in a particular carrier at aparticular concentration, there exists a specific film thickness belowwhich there is no fluorescence, and above which there is a fluorescenceresponse. The threshold thickness of the tracer film may be termed thedimensional sensitivity of the fluorescent tracer. The concept ofthin-film fluorescence is applicable to a fluorescent substance insolution in a solid form (e.g., as a plastic or resinous material) aswell as in liquid form.

As part of my discovery, it has been found that the dimensionalsensitivity of a fluorescent sensitizer is independent of the color orfluorescent brightness thereof. Instead, each fluorescent sensitizersubstance can be assigned a specific sensitivity value k, as will bedescribed hereinbelow, and the dimensional sensitivity is a function ofk and the concentration C of the sensitizer. The dimensional sensitivityof a given fluorescent sensitizer can, over a useful range ofconcentrations, be determined by a method to be described below. Inconjunction with such method, there may be measured, for eachfluorescent material in solution, a sensitivity index Is value, which isdefined in accordance with the following relationship:

The sensitivity index IE value of a fluorescent material in solution isa direct function of the threshold film thickness of said material, andserves as a convenient means of expressing the dimensional sensitivitythereof.

For the purpose of measuring the dimensional sensitivity of afluorescent material in solution, and also of determining the I,s valuethereof, I employ a Meniscus Method, which is described in my U.S. Pat.No. 3,107,298, granted Oct. l5, 1963, for Apparatus for the Measurementof Fluorescent Tracer Sensitivity. In practicing the invention disclosedin the latter patent, a flat `glass platen is positioned under a blacklight, and a drop of liquid having dissolved therein a fluorescentsubstance is placed on the platen. A convex lens having a preferredradius of curvature of 106 em. is then placed over the drop of liquid,and so as to rest on the platen. At the point of contact between thelens and the platen, the liquid film has a thickness of substantiallyzero, and a meniscusshaped film surrounds said contact point.

As pointed out in Pat. No. 3,107,298, the thickness of the liquid filmvaries continuously with the radial distance from the contact point.However, the fluorescence response as seen under black light is adistinctly discontinuous function, so that a nonfluorescent spot is seenin the region of the contact point. If the flat platen is made of ablack glass, the nonfluorescent spot appears as a black spot whichcontrasts sharply with the surrounding area of fluorescence, and whichcan be measured as to its diameter with good accuracy. The diameter ofthis black spot is used as a measure of the film thickness above whichthe fluorescence response occurs, and below which fluorescence ceases.For a given fluorescent sensitizer dissolved in a particular solventmaterial, the diameter of the black spot varies depending upon theConcentration of the fluorescent sensitizer.

Inasmuch as the transition of fluorescence response, with respect tofilm thickness, is really a continuous function, the toe of thetransition curve approaches zero response as the film thickness is madesmaller, but, theoretically at least, never actually reaches zero. As a.practical matter in locating the point corresponding to the so-calleddimensional threshold of fluorescence, a point on the transitioncharacteristic curve of the tracer is taken at a film thickness which isone-tenth (l/m) the film thickness at the midpoint of the transitioncurve, Where the brightness of fluorescence response falls to a valuebelow about 2% to 7% of the maximum brightness which appears in arelatively thick film of the tracer composition. In the above-mentionedMeniscus Method test, the diameter of the black spot would be taken asthe distance between the points where brightness response begins to risefairly steeply, at the 2% to 7% values of maximum brightness asindicated above.

In measuring the fluorescent characteristics of various materials,employing the above-described Meniscus Method, I have discovered thatfluoranthene (ClEHm) exhibits fluorescent characteristics which renderit useful in the establishment of a reference standard, against whichthe performance of a fluorescent material may be evaluated. Referring,now, to FIG. l, a five-cycle single logarithmic chart is shown having alogarithmic horizontal, or abscissa, scale representing Is values,ranging from .01 to 1000. The vertical, or ordinate, scale at the leftrepresents spot diameters as determined by the abovedescribed MeniscusMethod of measurement, and ranges from 0 to 4 mm. A straight line 5 isdrawn as shown, between two points having respectively an ordinate of 4and an abscissa of .01, and an ordinate of 0 and an abscissa of 1000.Said straight line provides a suitable reference characteristic forpurposes of standardization, which reference characteristic correspondsto an imaginary fluorescent substance having a specific sensitivity k ofunity 1.

Since the fluorescent properties of fluoranthene are closelyapproximated by those represented by the diagonal line 5, it is oftenconvenient to use fluoranthene as a reference material in evaluating thethreshold response characteristics of other sensitizer materials. Insuch usage, a series of tracer solutions of fluoranthene may be preparedhaving accurately measured concentrations of fluoranthene in a suitablesolvent carrier such as N-methyI-Z-pyrrolidone, as for Examples 2, 5,10, 15, 25, 40, 75, 100, 150, 200, etc., grams per pint.N-methyl-Z-pyrrolidone is mentioned as a suitable solvent forfluoranthene since it will dissolve this sensitizer to concentrationsgreater than 200 grams per pint at room temperature. The unit ofmeasure, grams per pint, is used for the reason that such values arenumerically equal to pounds per gallons, so that laboratory test resultsmay be quickly and conveniently translated for production of drumquantities of material.

If a series of lens and platen setups are prepared sideby-side underblack light using the above-described fluoranthene solutions, they willexhibit a graduated scale of spot diameters ranging from 3 mm. or moredown to as small as ..5 mm. or less. If, then, a test solution of agiven fluorescent sensitizer is prepared in a suitable solvent carrierand to a certain concentration, for example, 15 grams per pint, aMeniscus Method test of such solution will provide a characteristic spotdlameter which may be quickly estimated by a visual comparison with thefluoranthene scale, For example, if the test material, at aconcentration of 15 grams per pint yields a spot diameter similar toffuoranthene at 40 grams per pint, then the I value of the material is40 and its k value is 40+15=2.66. By use of the aboveoutlined procedure,it has been possible to rapidly evaluate the fluorescence responsecharacteristics of the many existing fluorescent dyes and other chemicalmaterials so as to select those which are useful as sensitizers for thepurpose of this invention, and which are listed hereinbelow.

It should be mentioned that fluorescence response characteristics, andlikewise the k value for a given sensitizer material, may vary dependingon the solvent carrier which is employed and the concentration level, sothat in cases where precision in measurement is wanted, it is desirableto specify the conditions under which the measurements are made. Formost practical purposes, adequate accuracy may be obtained bydetermining the k value for a sensitizer at a concentration of grams perpint using a solvent carrier which has sufficient solvent capability forthe sensitizer such that the concentration of 15 grams per pint is wellbelow the saturation point of the solution. A wide variety of solventmaterials may be employed, as will be described below.

At the right-hand side of the chart of FIG. 1, an ordinate scale,representing dimensional sensitivity (threshold film thickness) inmillimicrons, is provided. ln view of the geometry of the standardtesting apparatus employed, and particularly in View of the known radiusof curvature of the lens, the dimensional sensitivity (or threshold filmthickness) is mathematically calculated, being a direct function of thespot diameter.

As will be clear, the dimensional sensitivity of a fluorescent dyesolution is a basic physical characteristic thereof, and the MeniscusMethod represents a convenient means for the determination of the valuefor this characteristic, It should be mentioned that nonfluorescentvisible color dyes in solution also exhibit thin film dimensionalthresholds similar to the behaviour of fluorescent substances. However,noniluorescent dyes rarely if ever provide dimensional thresholdproperties smaller than 250 millimicrons, and their k values arevirtually always less than about .1. Accordingly, fluorescent materials,and particularly those which have k values above about .2, and which arethus useful for the purpose of this invention, are in a categorydistinct from ordinary nonfluorescent visible color dyes or coloringmaterials or ordinary low-sensitivity fluorescent dyes.

It will thus be understood that the dimensional sensitivity, orthreshold film thickness, of a fluorescent material is a function of thespecific sensitivity k of the sensitizer dye or other fluorescentsubstance employed, and of the concentration C of the latter. Theimportant significance of the foregoing discussion with regard todimensional sensitivity may be best understood by reerring now to FIG. 2which illustrates the behaviour of a fluorescent tracer of the inventionas used in the wellknown inspection penetrant process. A surface 7 has asmall flaw 8 in the form of a crack, a porosity, or an intercrystallineseparation. A liquid tracer, containing a fluorescent dye, is applied tosurface 7 such that it enters the flaw 8 and forms an entrapment 9. Thesurface 7 is cleaned to remove residual penetrant tracer, after whichthe liquid entrapment exudes from the flaw 8 to form a micro-thin film10 of liquid in the region of the surface flaw.

It will be understood that the amount of liquid which can exude from theflaw 8 depends on the size of the flaw, and likewise the thickness ofthe exuded tracer film will depend on the size of the surface flaw 8. Ifit 1s assumed that the thickness of the exuded film 10 is dimensionallyof the same order of magnitude as the size of the surface flaw 8, thenit follows that for an inspection penetrant to be capable of revealingthe presence of flaws having dimensions of the order of a halfwavelengthof light or 250 millimicrons, the dye tracer must have a dimensionalthreshold smaller than 250 millimicrons, and an Is value greater thanabout l5 as shown on the chart of FIG. l.

Referring now to FIG. 3, which illustrates the behaviour of a liquidtracer of the invention as used in a leak detection process, a wall 12of a tank or pipeline may be tested for liquid leaks through amicroscopic leakage path 13 by applying a dyed liquid tracer 14 on oneside of the wall 12 and allowing the dyed tracer to migrate through theleakage path 13, either by capillary action or by pressurizing theliquid. As the liquid tracer exudes from the leakage path 13, it mayyield a micro-thin film 15 surrounding the point of leakage.

In cases where a leakage condition is extremely small, it may requirequite a long time for a substantial thickness of the tracer film 15 tobe generated. Thus, the leak detection capability of a dyed liquid leaktracer may may be equated with the thin-film indication response of theleak tracer. It will be understood that a fluorescent leak tracer, suchas one which will yield a fluorescence response at film thickness of theorder of 250 millirnicrons as provided by the tracer liquids of thisinvention will be effective in the detection of micro-leaks which willbe undetectable with solutions of ordinary nonfluorescent dyes orordinary low-sensitivity fluorescent dyes.

Referring now to FIG. 4, a surface 17 is coated with a thin film ofplastic material 18 which is to act as an electrical insulating layer.For the purpose of this illustration, it is desired that the thicknessof the applied insulating layer shall be smaller than a half wavelengthof light or 25() millimicrons. At the same time, it may be desired thatthe presence of this thin insulating layer shall be revealed by itsfluorescent response. It will be seen, therefore, that in order for sucha layer to yield a fluorescence response, the Is value of the tracermaterial, of which the film consists, must be greater than about 15, asshown on the chart of FIG. 1.

Again referring to FIG. 4, the layer 18 may be formed by means of a waxyfilm from a crayon or by means of a marking ink. In soldering operationsor in various chemical processes, fluorescence tagged soldering fluxesor chemical process materials may form layers of unwanted residues whichare desired to be detected by means of their fluorescence response. Inany event, and where the applied film is to exhibit fluorescence at filmthicknesses smaller than 250 millimicrons, the Is value must be greaterthan about 15 as shown on the chart of FIG. 1.

Solutions of the fluorescent substances of this invention all exhibitthin-film fluorescence response thresholds at various concentrationscorresponding approximately to those represented by the diagonal line inFIG. 1. Inasmuch as an Is value of 15 corresponds to a dye concentrationof about 15 grams per pint, a class of particularly useful fluorescenttracers (i.e., those which yield iluoresoence response at filmthicknesses smaller than 250 millimicrons) may be defined as those whichcontain a fluorescent substance, as specified hereinbelow, within arange of proportional concentrations greater than at least about 15grams per pint up to the limit of solubility of the fluorescentsubstance.

The sensitizers employed in accordance with the invention are selectedso that each has a comparatively high k value, i.e., within theapproximate range of .2 to 3. Certain of the sensitizers are identifiedbelow in accordance with their standard designations in the Color Index(2nd 7 ed., 1956, vols. l to 4), published by the Society of Dyers andColourists, Dean House, Piccadilly-Bradford, Yorkshire, England; theAmerican Association of Textile Chemists and Colorists; and LowellTechnological Institute, Lowell, Mass., U.S.A. The k value, asdetermined by the comparison method described above, is indicated foreach sensitizer thus listed.

For commercial usage, the Color Index listing provides an accuratedesignation for a given uorescent dye. Certain other uorescentmaterials, which are not normally considered to be dyestuffs, may bedesignated according to their appropriate chemical structures, asillustrated hereinbelow, or by their commonly employed chemical names.The many useful uorescent dyes which are listed in the Color Index, andothers which are not so listed, are well documented in the patentliterature. It should be understood that not all fluorescent dyes arenecessarily useful for the purpose of the invention. This is becausecertain uorescent dyes may lack adequate solubility, or their k valuesmay be excessively low. Substances which are useful for the purpose ofthis invention may be listed in groupings as follows. Color Indexdesignations are given where applicable.

(a) Fluorescent (di)aminostilbine(di)sulfonic acid dyes-Most of thesedyes are characterized by k values in the range of from .2 to 3.

(b) Fluorescent dibenzothiophene dyes-Most of these dyes arecharacterized by k values in the range of from .2 to 3.

(c) Fluorescent monoazole dyes-These materials are characterized by kvalues in the range of from .2 to 3.

(d) Fluorescent bisazole dyes-These materials are characterized by kvalues in the range of from .2 to 3.

(e) Fluorescent coumarin dyes-These materials are characterized by kvalues in the range of from .2 to 3, usually above l.

(f) Fluorescent perylene dyes.-These materials are characterized by kvalues in the range of .2 to 3, usually around .25.

(g) Fluorescent naphthalic acid imide dyes- These materials arecharacterized by k values in the range of from 2 to 3.

(h) Fluorescent pyridotriazole dyes- These materials are characterizedby k values in the range from .2 to 3.

(i) Fluorescent di-hydrocollidine dyes- These mas.

terials are characterized by k values in the range of from .2 to 3,usually around .25.

(j) Fluorescent acridine dyes- These materials are characterized by kvalues in the range of from .2 to 3.

They are listed in the Color Index under the following Color Index (CJ.)designations.

C.I. Number:

C.I. Basic Orange 14, 15, 16, and 23. C.I. Basic Yellow 6 and 9.

(k) Fluorescent xanthene dyes-These materials are characterized by kvalues in the range of from .2 to 3, usually around .25. They are listedin the Color Index under the following Color Index designations.

C.I. Number:

C.I. Basic Red 8. CJ. Basic Violet 11.

(l) Fluorescent brightening agent dyes-These materials are listed invol. 4 of the Color Index under Fluorescent Brightening Agents (fortextile usage). The C.I.

B/A (Brightening Agent) numbers and applicable k values are indicated asfollows:

C.I. B/A 2, k=.25 C.I. B/A 5, k=.2 C.I. B/A 9, k=1.35 C.I. B/A 25, k=.25C.I. B/A 30, k=.2 C.I. B/A 53, k=l.35 C.I. B/A 56, le==.2 C I. B/A 60,k=1.0 C.I. B/A 66, k=.25 C.I. B/A 69, k=1.0 C.I. B/A 74, k=.5 C.I. B/A77, k=.5 C.I. B/A 3, k=1.0 C.I. B/A 6, k=.25 C.I. B/A 22, k .23 C.I. B/A26, k=1.7 C.I. B/A 46, k=2.3 C.I. B/A 54, k=1.35 C.I. B/A 57, k:1.35C.I. B/A 61, k=.65 C I. B/A 67, k=1.0 C.I. B/A 70, k=1.7 C.I. B/A 75,k=.5 C.I. B/A 78, k=2.65 C.I. B/A 4, k=.8 C.I. B/A 8, k=.5 C.I. B/A 24,k=.2 CI. B/A 29, k=.5 C.I. B/A 47, k=.65 C.I. B/A 55, k=1.7 C.I. B/A 59,k=.25 C.I. B/A 65, k=.2 Cl. B/A 68,k=1.7 C.I. B/A 7l, k=l.0

C.I. B/A 76, k=1.35

CJ. Basic Orange 10, k=.25 C I. Acid Yellow 7, k=.25 C I. Solvent Yellow44, k=.5 C.I. Disperse Yellow 3l, k=.2 C.I. Basic Red 1, k=.25

C.I. Solvent Red 36, k=.5

C.I. Acid Red 87, k=1.0

C.I. Solvent Red 45, k=1.0 C.I. Disperse Yellow 13, k\=.5 C.I. DirectYellow 59, k=2.65 C.I. Disperse Yellow l1, k=.25 C.I. Basic Yellow 7,k=1.7

C.I. C.I. C.I. C.I.

Basic Violet 10, k=.25 Acid Red 52, k=1.0 Acid Violet 7, k= 1.0 Acid Red50, k=.25 C.I. Developer 8, k=.5

C.I. Solvent Green 5, k=.25

(n) Fluorescent chemical compounds-The following materials arecharacterized by k values in the range of from .25 to 1.7:

1 '-Iciiihydroxy-Zaceton aphthone-HOC10H6COCH3,

Fluoranthene-CNHW represented by the structural formula:

Pyrene-CwHlo, represented by the structural formula:

In the formulation of the fluorescent tracers of the invention,color-formers may be employed as well as fluorescent sensitizers. Acolor-former is an ordinary fluorescent or noniiuorescent dye materialwhich has the ability to contribute color to or shift the fluorescentcolor of a dye mixture containing at least one sensitizer. Acolor-former, in other words, provides a characteristic color for thetracer. While the sensitizers of the invention have k values within theapproximate range of .2 to 3, color-formers or ordinary dyes usuallyhave k values somewhat less than .l (even though they may yield intensecoloration or high fluorescent brightness).

Many dyes are suitable for use as color-formers. These ordinary dyes orcoloring materials, which are also listed in the Color Index, may beused in conjunction with any of the above-listed sensitizers, the onlyrequirement being that the color-former must be selected for itssolubility in the same solvent carrier in which the particularsensitizer is dissolved, as well as for its color characteristic.

Most of the sensitizers which are grouped above, uoresce blue, bluishwhite, or bluish green, while a few uoresce yellow, orange, or red. Ithas been found that only in a comparatively few instance will afluorescent dye which is able to provide a substantially highdimensional sensitivity produce some color other than blue or green.

The sensitizers grouped above may be employed either singly, or incombination with one another. A prerequisite for combination of thesensitizers, of course, is that those combined shall each be compatiblewith a particular solvent system.

A wide variety of solvents may be utilized as carriers for theabove-designated fluorescent sensitizers. The following listing in TableI includes solvent materials, both liquid and resinous (or plastic)which have been tested with the foregoing fluorescent sensitizers. Inall cases, it has been found possible to obtain a desired dimensionalthreshold in the dye solution smaller than 250 millimicrons.

TABLE I--SOLVENT MATERIALS TESTED water acidied water alkaline waterethylene glycol diethylene glycol triethylene glycol polyethyleneglycols (M.W.-200 to 6060) ethylene glycol monobutyl ether diethyleneglycol monobutyl ether ethylene glycol monethyl ether diethylene glycol`monethyl ether polyoxyalkaline glycols (Ucon fluids) dimethyl formamideN-methyl-Z-pyrrolidone dimethyl sulfoxide N-vinyl-Z-pyrrolidone siliconoil (Corning 200) nitroethane polyester resin (Am. Cyanamid Laminac)alkyl resin (Amer. Cyan. Rezyl) rosin methanol ethanol isopropanolisodecanol acetone methyl ethyl ketone methyl isobutyl ketone diacetonealcohol mesityl oxide isophorone ethyl amyl ketone mineral thinner(naphtha) kerosene carbon tetrachloride perchloroethylene methylenechloride tiuorocarbon oil (Hooker Florolube) sodium petroleum sulfonateurea formaldehyde resin (Am. Cyan. Beetle) epoxy resin (Shell Epon)vinyl resin (Union Carbide VYGH) In addition to the above-listedsolvents, it has been found that the solvent carriers listed in mycopending application, Ser. No. 492,676, tiled Oct. 4, 1965, now issuedPat. No. 3,311,479, for Penetrant Inspection Process and Composition,[now abandoned] also function well as carriers for the uorescentsensitizers of this invention.

The choice of a solvent system for use in conjunction with a particularsensitizer of the invention, and the manner of effecting the solution ofthe sensitizer are accomplished in accordance with procedures well knownin the art. Thus, for example, a sensitizer which is normally relativelyinsoluble in mineral spirits may be coupled into solution with thelatter by means of a glycol ether. Likewise, a sensitizer which isrelatively insoluble in water may be coupled into a water solution bymeans of an alcohol additive or by use of a surfactant, such asethoxylated alkylphenol. Although aliphatic mineral solvents haverelatively low solvent capabilities for most of the sensitizers of theinvention, the solution may still often be effected through theadditional use of a small amount of aromatic mineral solvent, the lattermaterially increasing the ability of the liquid mixture to carry thesensitizer into solution.

It has been discovered that fluorescent materials produced in the mannerdescribed above may exhibit dimensional thresholds of fluorescence ofthe order of 2.5 X 10-5 to 2x10-6 cm. or less, if there is employed aproportional concentration of sensitizer of at least about 15 grams perpint. Referring again to FIG. 1, it will be noted that the value of 2.5105 cm., or 250 millimicrons, is equivalent to a sensitivity index Isvalue of 15. Thus, it will be understood that such an Is value of 15(within about one order of magnitude), or greater, is achieved upon thedissolution in a suitable solvent of about 15 grams per pint, or more,of any of the sensitizers of the group specified.

It will be understood that the dissolution of a sensitizer having a kvalue of .2 in the proportional concentration of 15 grams per pint willyield a sensitivity index I5 value of 3, which corresponds to adimensional threshold of fluorescence of about 500 millimicrons. Thissensitivity condition is well within one order of magnitude of a desiredsensitivity threshold characteristic of 250 millimicrons, and suchmaterials are, therefore, considered to be useful for the purpose of theinvention. For sensitizers which have k values as low as .2, it ismerely necessary to employ a proportional concentration of about 75grams per pint in order to achieve a desired dimensional sensitivity of250 millimicrons. Some of the sensitizers have exceptionally highsolubilities in particular solvents, and may permit use atconcentrations as high as about 300 grams per pint. Thus, a sensitizerhaving a k value of one (1) will, when used in a concentration of about15 grams per pint, provide approximately a value of Isi=kC=15, withequivalent dimensional sensitivity of the order of 2.5)(10-5 cm.However, if the sensitizer is iluoranthene, and the solvent isN-methyl-Z-pyrrolidone, the concentration may be increased up to a valueof the order of 300 grams per pint, with a corresponding increase indimensional sensitivity to a value of the order of 2 106 cm. or less.Certain fluorescent coumarin dyes, notably C.I. Fluorescent BrighteningAgents 68 and 69, also have excellent solubility in solvents such asN-methyl-Z-pyrrolidone, dimethyl formamide, or dimethyl sulfoxide, andare thus capable of providing dimensional sensitivities in the range of2 l06 cm. or less.

It may be noted that the minimum concentration of about 15 grams perpint, employed in accordance with the invention, is equivalent to about3.3% concentration by weight. This value is well above the maximumvalues for the concentrations of fluorescent dyes employed in the priorart, which, as indicated above, have been of the order of .5% to 2%. Themaximum allowable concentration depends only on the solubility of theparticular sensitizer in a given solvent carrier. All of the abovelisted sensitizers will dissolve in appropriate solvents toconcentrations of the order of 40 grams per pint or about 8% by weight,while a large number of the sensitizers will dissolve in suitablesolvents to concentrations of the order of 100 grams per pint or about20% by weight. A few of the sensitizers may be dissolved in appropriatesolvents to concentrations greater than about 300 grams per pint, orabout 60% by weight or more.

It should be pointed out that, although the limit of dimensionalsensitivity for most solutions of sensitizers is about ZXIO- to 4 l05cm., it is possible to extend the dimensional sensitivity of a givenfluorescent tracer well down to-ward 10|'7 cm. by means of a liquiddeveloper technique which is the subject of my now issued U.S. Pat. No.3,184,596, granted May 18, 1965, for Flaw Detection Method Using aLiquid Solvent Developer.

Primarily, as a result of the very low threshold lm thickness exhibitedby the fluorescent tracers of the invention, the use of said tracersoffers advantages not heretofore obtainable in industrial inspection,production control, quality assurance, and other similar applications.For example, in the case of jet engine turbine blades, micro-cracksoften occur which have dimensions on the order of 10-5 cm. In the caseof heat resistant surface coatings for space vehicles, porosity defectsmay occur which likewise have dimensions on the order of 10-5 Whenemploying a fluorescent penetrant tracer for aw detection purposes, theprocess in its simplest form includes dipping the part to be tested inthe penetrant,

draining the part, cleaning the surface penetrant by washing (and in thecase of non-washable penetrant, applying an emulsier-coupler dip priorto washing), and finally inspecting the part under black light. In somecases, an additional step of applying a so-called developer is employedfor the purpose of enhancing the fluorescent brightness of the awindications.

EXAMPLE NO. 1

As an example of the use of the Meniscus Method in connection with thechart of FIG. l, a particular uorescent substance (Pl/renc) present in aliquid solvent (ethylene glycol monethyl ether) at a concentration of 9grams per pint exhibited a spot diameter of 1.45 mm. The equivalentsensitivity index Is value corresponding to this spot diameter wasdetermined by reading on the chart across to the reference line 5, alongline x, and was found to be at line y. The specific sensitivity k of thesubstance (at this concentration) was then determined in accordance withthe relationship Is=kC. Thus, k=l5 divided by 9, or 1.7. The equivalentdimensional sensitivity, or threshold film thickness, was also read fromthe chart by following line x across to the right hand scale ofordinates, and was found to be equal to 250 millimicrons.

12 EXAMPLE No. 2

A penetrant tracer having the following formulation was prepared:

Ethylene glycol monobutyl ether 5 gal. Fluoranthene 15 lbs. C.I.Brightening Agent 1lb. 8 oz. Base oil pale 55 gal.

The above formulation is a typical medium-to-high sensitivity penetranttracer, insoluble in water. It is suitable, e.g., for detecting cracksin weldments, and has a dimensional sensitivity of about 180millimicrons.

EXAMPLE NO. 3

A penetrant tracer was prepared as follows:

The above formulation is a water washable penetrant tracer of highsensitivity, having a dimensional sensitivity of 143 millimicrons. It issuitable, among other uses, for the detection of micro-flaws in machineparts and ceramic materials.

EXAMPLE NO. 4

A penetrant tracer having the following formulation was prepared:

N-vinyl-2Fpyrrolidone gal l2 Fluoranthene lbs 75 C.I. Fluor. BrighteningAgent 75 lbs 5 Diethylene glycol monobutyl ether gal 4 Polyethyleneglycol di-Z-ethylhexoate, to 55 gal.

The above formulation is non-water-soluble, and has a dimensionalsensitivity of about 75 millimicrons. This tracer is particularlysuitable for use in extremely high sensitivity application, in whichtests are made for intergranular corrosion in metals, or fractures incrystal structures and the like.

EXAMPLE NO. 5

A penetrant tracer formulation was prepared as follows:

C.I. Fluor. Brightening Agent 46 28 lbs. C.I. Acid Yellow 7 3 lbs. 8 oz.Triethylene glycol, to 55 gal.

The above formulation is water soluble, having a dimensional sensitivityof about 75 millimicrons. It is suitable for high sensitivity tests onparts employed in liquid oxygen systems. As will be noted, theformulation meets the requirement that such systems be maintained freefrom oil contamination.

EXAMPLE NO. 6

A penetrant tracer having the following formulation was prepared:

Diethylene glycol monobutyl ether gal 9.2 Fluorantheue lbs 25 C.I.Fluor. Brightening Agent 75 lbs 5 Polyethylene glycol di2ethylhexoate,to 55 gal.

The above formulation is a high sensitivity, non-watersoluble penetranttracer having a dimensional sensitivity of about millimicrons. It iswell suited for the penetrant inspection and detection of extremelysmall surface aws.

Flux formulations of the invention may be employed in processes fordetecting defects in soldered joints. Such 13 processes usually include,as a primary step prior to inspection under black light, the removal ofall excess fluorescence-tagged solder flux from the surface of the partto be examined. A suitable cleaner is used for this purpose, Such aswater in the case of water-soluble fluxes, or alcohol in the case ofrosin-type fluxes. After the removal of the excess surface flux, therewill remain minute entrapments of flux in cracks, or in interfacialareas where there is incomplete fusion between solder and base metal.Such small entrapments of the fluorescent llux sometimes can be viewedsimply by exposure to black light. However, it may often be founddesirable to enhance the fluorescent indications by ap plying a liquiddeveloper of the aforementioned type to the soldered area. Such adeveloper will carry the fluorescent ilux tracer into solution and allowit to diffuse out into a region where it can become easily seen.

EXAMPLE NO. 7

A soldering flux was prepared as follows:

Glutamic acid hydrochloride 31 lbs. Urea 18 lbs. C.l. Fluor. BrighteningAgent 46 27 lbs. C.I. Acid Yellow 7 3 lbs. 8 oz.

Polyethylene glycol (molecular weight 200) to 5 gal.

EXAMPLE N0. 8

A soldering flux was prepared in accordance with the followingformulation:

Water white rosin grams-- 25 Fluoranthene do 25 Glutamic acidhydrochloride do 2.5 N-vinyl-2-pyrrolidone cc 20 Diacetone alcohol cc-..175

The above soldering flux is of the rosin type and of high sensitivity.It has an initial dimensional sensitivity of about 110 millimicrons, andan ultimate dimensional sensitivity of about 20 millirnicrons.

EXAMPLE NO. 9

A fluorescent tracer was prepared as follows:

Percent Cl. Fluor. Brightening Agent 26 15 C.I. Acid Violet 7(color-former) 1 Diethylene glycol, to 100.

The above formulation provides an ink suitable for use in industrialmarking, ballot marking, or as a business machine ink. lt has adimensional sensitivity of about 95 millimicrons. Such sensitivity levelis essential for adequate performance of the ink, since, as may beunderstood, the latter must reveal its fluorescence even though in theform of an extremely thin film. Although a red ink is here provided, itwill be realized that other colors, such as yellow or blue, can beobtained through change of the color-former dye.

EXAMPLE NO. l0

A llourescent tracer concentrate having the following formulation wasprepared:

14 C.I. Fluor. Brightening Agent 26 oz-- l Water pintS-- 1 Ethyleneglycol monoethyl ether do 5 Dissolve the above and then add:

C.I. Fluor. Brightening Agent 57 oz 4 Ethylene glycol monoethyl ether,to 1 gal.

The above concentrate is liquid, and is suitable for use in pools,fountains, or waterfalls, for artistic display effects. When used, theconcentrate is mixed with a suitable quantity of water. For example,pastel lluo rescent shades are obtained by a dilution ratio of about1500 to l, medium shades are obtained by a dilution ratio of 1000 to 1,and deep shades of fluorescent color are obtained by a dilution ratio of500 to 1. The above dye concentrate provides a blue fluorescent color.In this case, the C.I. Fluor. Brightening Agent 57 is used as acombination sensitizer and color-former dye. Others colors may, ofcourse, be provided through the use of different or additionalcolor-former dyes.

EXAMPLE NO. ll

A fluorescent tracer material having the following formulation wasprepared:

The above formulation provides a tracer material in dry form, preferredfor such uses as the marking of snowed-in airport runways, or as a seamarker for indicating the location of survivors at sea. The powders areblended throughly and packaged in moisture-tight containers. The salt isincluded for the purpose of causing ice or snow to melt so as to allowthe fluorescent dyes to go into solution. The silica aerogel serves torender the product fluffy and light, so that it will spread readily overa large area of water, snow, or ice, This formulation exhibits abrilliant fluorescent orange color when in water solution. The color canbe changed to a bright red by replacing the fluorescent with eosine, andit can be changed to a brilliant yellow-green by replacing the eosinewith fluorescein. When this marker tracer is dissolved in water, ityields a dimensional sensitivity of the order of millimicrons orsmaller.

EXAMPLE NO. l2

A fluorescent tracer having the following formulation was prepared:

C.I. Fluor. Brightening Agent 26 oz-- 8 Water, to make 1 gal.

The above formulation is a fluorescent blue concentrate having adimensional sensitvity of about 30 millimicrons, and being suitable foruse as a leak tracer for water solutions. When so used, it shouldusually be diluted in water (e.g., in the proportion of 1 to 200). Itsfluorescent color may be shifted to a bright green by the addition of agreen color-former, such as C.I. Acid Yellow 73 (Fluorescein) EXAMPLENO. 13

A crayon having the following formulation was prepared:

Polyethylene glycol (wax) 3 lbs. 12 oz. CJ. Fluor. Brightening Agent 263 oz. C.I. No. 42735-Acid Blue 104 (colorformer) 3 oz. White barytes 3lbs.

In forming the above crayon, the polyethylene glycol (wax) was meltedand raised to a temperature of about 150 F. The dyes and fillers werethen added, with the aid of a high speed disperser, and dispersion wascontinued until blending was complete. The hot mixture was then pouredinto crayon molds. The resulting uorescent wax had a dimensionalsensitivity of about 60 millimicrons. The crayon was found useful forsuch purposes as marking radar plotting boards, maps, ballots, etc.

EXAMPLE NO. 14

A fluorescent plastic composition for use as a surface coating wasprepared having the following percentage composition:

Percent Fluoranthene 14 C.I. Fluor. Brightening Agent 75 1.5

Polyester resin (Laminac No. 4110, trademark,

American Cyanamid Co.) 84.5

To the above resin composition was added as a catalyst, 80 cc. pergallon of methyl ethyl ketone peroxide. The mixture was applied in athin film between two glass plates, and cured to a hard material havinga bright green fluorescence, and having a dimensional sensitivity ofabout 90 millimicrons. A grainless fluorescent screen was thus provided,capable of converting to visible light ultra-violet images focusedthereon. This screen may serve as a converter reticle, and is useful invarious applications requiring the visual observation of ultra-violetimages, e.g., microscopes, or converter cells in communication systemsemploying modulated ultra-violet radiation.

EXAMPLE NO. 15

A fluorescent tracer having the following composition was prepared:

C.I. Fluor. Brightening Agent 68 gm 120 Eethoxylated nonylphenol molsethylene oxide per mol nonylphenol) gal-- 1/2 Water, to make 1 gal.

The above formulation is a fluorescent blue liquid suitable for markingtextiles for cutting or sewing in the manufacture of garments. It isespecially useful on dark colored fabrics which require tracers having ahigh level of sensitivity in order to reveal fluorescence. It has adimensional sensitivity of about 200 millimicrons or less.

EXAMPLE NO. 16

A fluorescent tracer having the following composition was prepared:

C I. Fluor. Brightening Agent 69 grams 128 Glycerin, to make l gal.

The above formulation has a bright blue fluorescence, and exhibits adimensional sensitivity on the order of 200 millimicrons or less. It isparticularly useful for application to rubber sealer gaskets, such asrefrigerator seals, from which it marks off onto a mating part to showup any discrepancies in the flt of the gasket.

EXAMPLE NO. 17

A fluorescent tracer concentrate having the following composition wasprepared:

C I. Fluor. Brightening Agent 46 lbs 2 Propylene glycol, to make l gal.

This is a fluorescent blue material which may be used as an additive towater soluble soldering fluxes. A pro portional concentration ispreferred of about 4 oz. of the tracer concentrate per gallon of liquidsoldering flux.

When used in this proportion, the tracer concentrate provides adimensional threshold thickness of fluorescence of less than 250millimicrons, suitable for the detection of unwanted residues of solderflux which may remain after a cleaning operation.

To facilitate the formulation of fluorescent tracer compositions inaccordance with the invention, while allowing for the use of any of avariety of solvents which may be encountered, a kit can be preparedcontaining an assortment of sensitizer substances, together with anassortment of color-former dyes. The sensitizers and color-former dyesmay be selected so as to provide a good range of typical materials. Inusing such a kit, it is a relatively simple matter to select asensitizer which is compatible with a particular solvent system `beingtested. It is then quite easy to select a suitable colorformer dye, ifsuch is required, to adjust the dye concentrations to their optimumvalues, and to add any other ingredients, such as thickeners, fillers,or the like.

Although the invention has been described with reference to particularembodiments thereof, it will be understood that various changes may bemade therein without departing from the spirit of the invention or thescope of the appended claims.

I claim:

1. In an inspection process in which thin fllms of a fluorescent tracerare revealed by a fluorescence response, the step of [preparing afluorescent tracer by dissolving at least one sensitizer in a solventcarrier] applying a fluorescent tracer to a test surface, saidfluorescent tracer consisting essentially of at least one sensitizerselected from the group consisting of fluorescent (di)aminostilbine(di)sulfonic acid dyes, fluorescent dibenzothiophene dyes, fluorescentmonoazole dyes, fluorescent bisazole dyes, fluorescent coumarin dyes,fluorescent perylene dyes, fluorescent naphthalic acid imide dyes,fluorescent pyridotriazole dyes, fluorescent di-hydrocollidine dyes,pyrene, 1'hydroxy-2'-acetonaphthone, fluorescent acridine dyes havingColor Index designations of 46000, 46025, 46050, 46055, 46060, 46065,46080, Basic Orange 14, l5, 16, 23, and Basic Yellow 6 and 9,fluorescent xanthene dyes having Color Index designations of 45000,45005, 45006, 45010, 45015, 45ml), 45050, 45090, 45100, 45105, 45155,45165, 45210, Basic Red 8, and Basic Violet 11, fluorescent brighteningagents having Color Index designations of C I. Brightening Agents 2, 3,4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59,60, 61, 65, 66, 67, 68, 69, 70, 71, 74, 75, 76, 77, 78, and fluorescentdyes having Color Index designations of Basic Orange 10, Acid Yellow 7,Basic Yellow 7, Disperse Yellow 11, 13, and 31, Direct Yellow 59,Solvent Yellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Redl, Basic Violet l0, Acid Violet 7, Developer 8, and Solvent Red 36 and45, respectively, and a liquid solvent carrier for said sensitizer, saidsensitizer being selectively used singly and in combination and being[present in said] in solution in the liquid solvent carrier within therange of proportional concentrations from at least about 15 grams perpint up to the limit of solubility of said sensitizer, whereby thedimensional threshold of fluorescence is adjusted to an operationalvalue below about 250 millimicrons.

2. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent brightening agentshaving Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57,68, 69, 75, 78, and dyes having Color Index designations of 49010 and56205, respectively, and a solvent carrier for said sensitizer, saidsensitizer being selectively used singly and in combination and beingpresent in said solvent carrier within the range of proportionalconcentrations from at least about 15 grams per pint up to the limit ofsolubility of said sensitizer, whereby the dimensional threshold offluorescence is adjusted to an operational value below about 250millimicrons.

3. An inspection process in accordance` with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent brightening agentshaving Color Index designations of `8, 22, 24, 25, 26, 29, 46, 53, 57,68, 69, 75, 78, and

dyes having Color Index designations of 49010 and 56205, respectively, acolor-former dye imparting a characteristic color to said tracer, and asolvent carrier for said sensitizer, said sensitizer being selectivelyused singly and in combination and being present in said solvent carrierwithin the range of proportional concentrations from at least about 15grams per pint up to the limit of solubility of said sensitizer, wherebythe dimensional threshold of lluoresence is adjusted to an operationalvalue below about 250 millimicrons.

4. An inspection process in accordance with claim l in which saidlluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of lluorescent brightening agentshaving Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57,68, 69, 75, 78, and dyes having Color Index designations of 49010 and56205, respectively, and a liquid solvent carrier for said sensitizer,said sensitizer being selectively used singly and in combination andbeing present in said liquid solvent carrier within thc range ofproportional concentrations from at least about 15 grams per pint up tothe limit of solubility of said sensitizer, whereby the dimensionalthreshold of fluorescence is adjusted to an operational value belowabout 250 millimicrons.

5. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent brightening agentshaving Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57,68,69, 75, 78, and dyes having Color Index designations of 49010 and 56205,respectively, a color-former dye imparting a characteristic color tosaid tracer, and a liquid solvent carrier for said sensitizer, saidsensitiaer being selectively used singly and in combination and beingpresent in said liquid solvent carrier within the range of proportionalconcentrations from at least about l5 grams per pint up to the limit ofsolubility of said sensitizer, whereby the dimensional threshold offluorescence is adjusted to an operational value below about 250millimicrons.

6. An inspection process in accordance with claim l in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent brightening agentshaving Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57,68, 69, 75, 78, and dyes having Color Index designations of 49010 and56205, respectively, and a resinous solvent carrier for said sensitizer,said sensitizer being selectively used singly and in combination andbeing present `in said resinous solvent carrier within the range ofproportional concentrations from at least about l5 grams per pint up tothe limit of solubility of said sensitizer, whereby the dimensionalthreshold of lluoresoence is adjusted to an operational value belowabout 25 0 millimicrons.

7. An inspection process in accordance with claim l in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent brightening agentshaving Color Index designations of 8, 22, 24, 25, 26, 29, 46, 53, 57,68, 69, 75, 78, and dyes having Color Index designations of 49010 and 56205, respectively, a color-former dye imparting a characteristic colorto said tracer and a resinous solvent carrier for said sensitizer, saidsensitizer being selectively used singly and in combination and beingpresent in said resinous solvent carrier within the range ofproportional concentrations from at least about grams per pint up to thelimit of solubility of said sensitizer, whereby the dimensionalthreshold of lluorescence is adjusted to an operational value belowabout 250 millimicrons.

8. An inspection process in accordance with claim 1 in which saidlluorescent tracer consists essentially or lluoranthene as a sensitizer,and a solvent carrier for said lluoranthene, said lluoranthene beingpresent in said solvent carrier within the range of proportionalconcentrations of at least about grams per pint up to the limit ofsolubility of said lluoranthene, whereby the dimensional threshold offluorescence is adjusted to an operational value below about 250millimicrons.

9. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of lluoranthene as a sensitizer,a color-former dye imparting a characteristic color to said tracer, anda solvent carrier for said lluoranthene, said fluoranthene being presentin said solvent carrier within the range of proportional concentrationsof at least about 25 grams per pint up to the limit of solubility ofsaid lluoranthene, `whereby the dimensional threshold of fluorescence isadjusted to an operational value below about 250 millimicrons.

10. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of pyrene as a sensitizer, and asolvent carrier for said pyrene, said pyrene being present in saidsolvent carrier `within the range of proportional concentrations of atleast about 15 grams per pint up to the limit of solubility of saidpyrene, whereby the dimensional threshold of fluorescence is adjusted toan operational value below about 250 millimicrons.

11. An inspection process in accordance with claim l in which saidfluorescent tracer consists essentially of pyrene as a sensitizer, acolor-former dye imparting a characteristic color to said tracer, and asolvent carrier for said pyrene, said pyrene being present in saidsolvent carrier within the range of proportional concentrations of atleast about 15 grams per pint up to the limit of solubility of saidpyrene, whereby the dimensional threshold of fluorescence is adjusted toan operational value below about 250 millimicrons.

12. An inspection process in accordance with claim 1 in iwhich saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of pyrene;1'-hydroxy-2-acetonaphthone; fluorescent acridinc dyes having ColorIndex designations of 46000, and 46025, 46050, 46055, 46060, 46065,Basic Orange 14, 15, 16, 23, and Basic Yellow 6 and 9; lluorescentxanthene dyes having Color Index designations of 45000, 45005, 45006,45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165,45210, Basic Red 8, and Basic Violet l1; lluorescent brightening agentshaving Color `Index designations of C.I. Brightening Agents 2, 3, 4, 5,6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61,65, 66, 67, 68, 69, 70, 71, 74, 75, 76, 77, 78; and fluorescent dyeshavin Color Index designations of Basic Orange 10, Acid Yellow 7` BasicYellow 7, Disperse Yellow 11, 13, and 31, Direct Yellow 59, SolventYellow 44, Solvent Green 5, Acid Red 50. 52, and 87, Basic Red l, BasicViolet l0, Acid Violet 7, Developer 8, and Solvent Red 36 and 45,respectively, and a solvent carrier for said sensitizer, said sensitizerbeing selectively used singly and in combination and being present insaid solvent carrier within the range of proportional concentrationsfrom at least about 15 grams per pint up to the limit of solubility ofsaid sensitizer, whereby the dimensional threshold of fluorescence isadjusted to an operational value below about 250 millimicrons.

13. An inspection process in accordance with claim l in `which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of pyrene;1'-hydroxy-2'acetonaphthone; fluorescent acridine dyes having ColorIndex designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080,Basic Orange 14, 15, 16, and 23. and Basic Yellow 6 and 9; fluorescentxanthene dyes having Color Index designations of 45000, 45005, 45006,45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165,45210. Basic Red 8, and Basic Violet 11; lluorescent brightening agentshaving Color Index designations of Cl. Brightening Agents 2, 3, 4, 5, 6,8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61,65, 66, 67, 68, 69, 70, 7l, 74,75, 76, 77, and 78; and fluorescent dyeshaving Color Index designations of Basic Orange 10, Acid Yellow 7, BasicYellow 7, Disperse Yellow l1, 13, and 31, Direct Yellow 59,

Solvent Yellow 44, Solvent Green 5, Acid Red 50, 52, ard 87, Basic Redl, Basic Violet 10, Acid Violet 7, Developer 8, and Solvent Red 36 and45, respectively, a color-former dye imparting a characteristic color tosaid tracer, and a solvent carrier for said sensitizer, said sensitizerbeing selectively used singly and in combination and being present insaid solvent carrier within the range of proportional concentrationsfrom at least about 15 grams per pint up to the limit of solubility ofsaid sensitizer, whereby the dimensional threshold of fluorescence isadjusted to an operational value below about 250 millimicrons.

14. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluoranthene; pyrene;1'-hydroxy-2'acetonaphthone; fluorescent acridine dyes having ColorIndex designations f 46000, 46025, 46050, 46055, 46060, 46065, 46080,Basic Orange 14, 15, 16 and 23, and Basic Yellow 6 and 9; fluorescentxanthene dyes having Color Index designations of 45000, 45005, 45006,45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165,45210, Basic Red 8 and Basic Violet 11; fluorescent brightening agentshaving Color Index designations of C.I. Brightening Agents 2, 3, 4, 5,6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61,65, 66, 67, 68, 69, 70` 7l, 74, 75, 76, 77, 78; and fluorescent dyeshaving Color Index designations of Basic Orange 10, Acid Yellow 7, BasicYellow 7, Disperse Yellow 11, 13, and 31, Direct Yellow 59, SolventYellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red 1, BasicViolet 10, Acid Violet 7, ,Developer 8, and Solvent Red 36 and 45,respectively, and a liquid solvent carrier for said sensitizer, saidsensitizer being selectively used singly and in combination and beingpresent in said liquid solvent carrier within the range of proportionalconcentrations from at least about 25 grams per pint up to the limit ofsolubility of said sensitizer, whereby the dimensional threshold offluorescence is adjusted to an operational value below about 250millimicrons.

15. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluoranthene; pyrene;1'-hydroxy-2'-acetonaphthone; fluorescent acridine dyes having ColorIndex designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080,Basic Orange 14, l5, 16 and 23, and Basic Yellow 6 and 9; fluorescentxanthene dyes having Color Index designations of 45000, 45005, 45006,45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165,45210, Basic Red 8, and Basic Violet 11; fluorescent brightening agentshaving Color Index designations 0f C.I. Brighlening agents 2, 3, 4, 5,6, 8, 9, 22, 24, 25, 26, 29, 30,46, 47, 53, 54, 55, 56, 57, 59, 60, 61,65, 66, 67, 68, 69, 70, 71, 74, 75, 76, 77, 78; and flourescent dyeshaving Color Index designations of Basic Orange l0, Acid Yellow 7, BasicYellow 7, Disperse Yellow 11, 13, and 31, Direct Yellow 59, SolventYellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red 1, BasicViolet l0, Acid Violet 7, Developer 8, and Solvent Red 36 and 45,respectively, a color-former dye imparting a characteristic color tosaid tracer, and a liquid solvent carrier for said sensitizer, saidsensitizer being selectively used singly and in combination and beingpresent in said liquid solvent carrier within the range of proportionalconcentrations from at least about 25 grams per pint up to the limit ofsolubility of said sensitizer, whereby the dimensional threshold offluorescence is adjusted to an operational value below about 250millimicrons.

16. An inspection process in accordance with claim l in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting 0f fluoranthene; pyrene;l'-hydroxy-Z'-acetonaphthone; fluorescent acridine dyes having ColorIndex designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080,Basic Orange 14, 15, 16, and 23, and Basic Yellow 6 and 9; fluorescentxanthene dyes having Color Index designations of 45000, 45005, 45006,45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165,45210, Basic Red 8, and Basic Violet l1; fluorescent brightening agents2, 3, 4, 5, 6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 53, 54, 55, 56, S7, 59,60, 61, 65, 66, 67, 68, 69, 70, 71, 74, 75, 77, 78; and fluorescent dyeshaving Color Index designations of Basic Orange 10, Acid Yellow 7, BasicYellow 7, Disperse Yellow l1, 13, and 3l, Direct Yellow 59, SolventYellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red 1, BasicViolet l0, Acid Violet 7, Developer 8, and Solvent Red 36 and 45,respectively, and a resinous solvent carrier for said sensitizer, saidsensitizer being selectively used singly and in combination and beingpresent in said resinous solvent carrier within the range ofproportional concentration from at least about 25 grams per pint up tothe limit of solubility of said sensitizer, whereby the dimensionalthreshold of fluorescence is adjusted to an operational value belowabout 250 millimicrons.

17. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluoranthene; pyrene;1'-hydroxy-Z'-acetonaphthone; fluorescent acridine dyes having ColorIndex designations of 46000, 46025, 46050, 46055, 46060, 46065, 46080,Basic Orange 14, l5, 16 and 23; and Basic Yellow 6 and 9; fluorescentxanthene dyes having Color Index designations of 45000, 45005, 45006,45010, 45015, 45020, 45050, 45090, 45095, 45100, 45105, 45155, 45165,45210, Basic Red 8, and Basic Violet 11; fluorescent brightening agentshaving Color Index designations of C.I. Brightening agents 2, 3, 4, 5,6, 8, 9, 22, 24, 25, 26, 29, 30, 46, 47, 53, 54, 55, 56, 57, 59, 60, 61,65, 66, 67, 68, 69, 70, 7l, 74, 75, 76, 77, 78; and fluorescent dyeshaving Color Index designations of Basic Orange 10, Acid Yellow 7, BasicYellow 7, Disperse Yellow 1l, 13, and 31, Direct Yellow 59, SolventYellow 44, Solvent Green 5, Acid Red 50, 52, and 87, Basic Red l, BasieViolet 10, Acid Violet 7, Developer 8, `and Solvent Red 36 and 45,respectively a color-former dye imparting a characteristic color to saidtracer, and a resinous solvent carrier for said sensitizer, saidsensitizer being selectively used singly and in cornbination and beingpresent in said resinous solvent carrier within the range ofproportional concentrations from at least about 25 grams per pint up tothe lirnit of solubility of said sensitizer, whereby the dimensionalthreshold of fluorescence is adjusted to an operational value belowabout 250 millimicrons.

18. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent(di)aminostilbine(di)sulfonic acid dyes, fluorescent dibenzothiophenedyes, fluorescent monoazole dyes, fluorescent bisazole dyes, fluorescentcournarin dyes, fluorescent perylene dyes, fluorescent naphthalic acidimide dyes, fluorescent pyridotriazole dyes, and fluorescentdihydrocollidine dyes, respectively, and a solvent carrier for saidsensitizer, said sensitizer being selectively used singly and incombination and being present in said solvent carrier within the rangeof proportional concentrations of from at least about 15 grams per pintup to the limit of solubility of said sensitizer, whereby thedimensional threshold of fluorescence is adjusted to an operationalvalue below about 250 millirnicrons.

19. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent(di)aminostilbine(di)sulfonic acid dyes, fluorescent dibenzothiophenedyes, fluorescent monoazole dyes, fluorescent bisazole dyes, fluorescentcoumarin dyes, fluorescent perylene dyes, fluorescent naphthalic acidimide dyes, fluorescent pyridotriazole dyes, and fluorescentdi-hydrocollidine dyes, respectively, a color-former dye imparting acharacteristic color to said tracer, and a solvent carrier for saidsensitizer, said sensitizer being selectively used singly and incombination and `being present in said solvent carrier within the rangeof proportional concentrations of from at least about 15 grams per pintup to the limit of solubility of said sensitizer, whereby thedimensional threshold of iluorescence is adjusted to an operationalvalue below about 250 millimicrons.

20. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of luorescent(di)aminostilbine(di)sulfonic acid dyes, iluorescent dibenzothiophenedyes, fluorescent monoazole dyes, uorescent bisazole dyes, fluorescentcoumarin dyes, fluorescent perylene dyes, fluorescent naphthalic acidimide dyes, iluorescent pyridotriazole dyes, and tluorescentdi-hydrocollidine dyes, respectively, and a liquid solvent carrier forsaid sensitizer, said sensitizer being selectively used singly and incombination and being present in said liquid solvent carrier Wihin therange of proportional concentrations of from at least about 25 grams perpint up to the limit of solubility of said sensitizer, whereby thedimensional threshold of iluorescence is adjusted to an operationalvalue below about 250 millimicrons.

21. An inspection process in accordance with claim 1 in which saidiluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent(di)aminostilbine(di)sulfonic acid dyes, lluorescent dibenzothiophenedyes, iluorescent monoazole dyes, tluorescent bisazole dyes, fluorescentcoumarin dyes, iluorescent perylene dyes, iluorescent naphthalic aciddyes, fluorescent pyridotriazole dyes, and fluorescent dihydrocollidinedyes, respectively, a color-former dye imparting a characteristic colorto said tracer, and a liquid solvent carrier for said sensitizer, saidsensitizer being selectively used singly and in combination and beingpresent in said liquid solvent carrier within the range of proportionalconcentrations of from at least about 25 grams per pint up to the limitof solubility of said sensitizer, whereby the dimensional threshold oflluorescence is adjusted to an operational value below about 250millimicrons.

22. An inspection process in accordance with claim 1 in which saidfluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent(di)aminostilbine(di)sulfonic acid dyes, fluorescent dibenzothiophenedyes, uorescent monoazole dyes, fluorescent bisazole dyes, iluorescentcoumarin dyes, tluorescent perylene dyes, fluorescent naphthalic aicidimide dyes, fluorescent pyridotriazole dyes, and fluorescentdi-hydrocollidine dyes, respectively, and a resinous Solvent carrier forsaid sensitizer, said sensitizer being selectively used singly and incombination and being present in said resinous solvent carrier withinthe range of proportional concentrations of from at least about 25 gramsper pint up to the limit of solubility of said sensitizer, whereby thedimensional thershold of fluorescence is adjusted to an operationalvalue below about 250 millimicrons.

23. An inspection process in accordance with claim 1 in which saidtluorescent tracer consists essentially of at least one sensitizerselected from the group consisting of fluorescent(di)aminostilbine(di)sulfonic acid dyes, tluo rescent dibenzothiophenedyes, iluorescent monoazole dyes, fluorescent bisazole dyes, tluorescentcoumarin dyes, fluorescent perylene dyes, fluorescent naphthalic acidamide dyes, fluorescent pyridotriazole dyes, and fluorescentdi-hydrocollidine dyes, respectively, a colorformer dye imparting acharacteristic color to said tracer, and a resinous solvent carrier forsaid sensitizer, said sensitizer being selectively used singly and incombination and being present in said resinous solvent carrier withinthe range of proportional concentrations of from at least about 25 gramsper pint to the limit of solubility of said sensitizer, whereby thedimensional threshold of lluorescence is adjusted to an operationalvalue below about 250 millimicrons.

References Cited The following references, cited by the Examiner, are ofrecord in the patent le of this patent or the original patent.

UNITED STATES PATENTS 2,733,216 1/1956 Eichholz et al. 252-30122,953,530 9/1960 Switzer 252-3012 3,108,187 10/1963 Thornbury 252-301.?.

FOREIGN PATENTS 569,920 6/1945 Great Britain. 143,492 8/1961 U.S.S.R.

OTHER REFERENCES Dement: Fluorochemistry, 1945. pp. 160, 187, 194 and198.

TOBIAS E. LEVOW, Primary Examiner A. P. DEMERS, Assistant Examiner U.S.Cl. X.R. 250-71, 71.5

